Several dose-levels of methadone will be administered to rats on the last two weeks of gestation to study the behavioral effects in the offspring. Standard procedures will be used to determine maternal toxicity, embryotoxicity, birthweight, growth and development. Studies of the acute hyperactivity that has been observed between two and three weeks of age among rats exposed prenatally to methadone will include a developmental study of activity level from birth to 30 days of age and pharmacological and biochemical studies of possible underlying mechanisms. To test the possibility that the hyperactivity is associated with the persistence of pharmacologically active amounts of methadone, radioactive methadone will be measured in brain and liver at 17 days of age. If found, we will attempt to precipitate opiate abstinence by the administration of naloxone. To investigate effects on behavioral development and learning ability in pre-weanling offspring, suckling behavior will be tested from birth to weaning and 20 and 30 day old offspring will be trained to perform an S plus, S minus discrimination task in a straight-alley maze. To study the long-term behavioral effects that have been observed in adulthood, offspring will be tested on DRL operant reinforcement schedules.